Refrigeration defrosting system



Sept. 8, 1959 J. R. BOYLE ETAL REFRIGERATION DEFROSTING SYSTEM FiledSept 9, 1955 3 Sheets-Sheet l 4 INVENTORS. (john 123.30 16,

BY flay .Gold,

Sept. 8, 1959 J. R. BOYLE ETAL 2,902,835

" REFRIGERATION DEFROSTING SYSTEM Fi led Sept. 9, 1955 Y a Sheets-Sheet2 L6 INVENTORS.

JUhW/JZ. 30 Le,- BY flmdflzd Sept. 8, 1959 J. R. BOYLE EIALREFRIGERATION DEFROSTING SYSTEM Filed Sept. 9, 1955 5 Sheets-Sheet 3INVENTORS. cjb7Z7'I/.R.B0 Ze BY flaz/zcifi bld 7 United States Patent2,902,835 REFRIGERATION DEFROSTING SYSTEM John R. Boyle, Chicago, Ill.,and David H. Gold, Hammond, Ind., assignors, by mesne assignments, toBohn Aluminum & Brass Corporation, Detroit, Mich, a corporation ofMichigan Application September 9, 1955, Serial N 0. 533,399

7 Claims. (CI. 62-45) This invention relates generally to defrostingsystems and more particularly to an automatic defrosting system forremoving the accumulation of frost on low temperature refrigerationcoils.

Various types of defrosting systems have been proposed for refrigerationcoils to automatically defrost the coils to remove ice therefrom so thatthe coils will operate efficiently. The refrigerating material used tocool the coils is sometimes heated for defrosting the same, and separateheating means such as electrical heaters are used in some cases todefrost the coils. However, these systems which have been available havenot been entirely satisfactory especially for use with coils operatingat very low temperatures. This is because a large amount of heat isrequired to remove ice which accumulates on the refrigeration coil.Accordingly, the coil must be heated for a long period of time and thismay result in deterioration of the items normally kept cool by therefrigeration coil.

It is an object of the present invention to provide a defrosting systemfor low temperature coils which produces sufficient heat to defrost thecoil in a short period of time.

Another object of the invention is to provide a defrosting system whichprovides the heat required for defrosting a low temperature coil in anefficient manner without producing excessive heat which might be appliedto items cooled by the coil to damage the same.

A further object of the invention is to provide a defrosting systemproviding efficient defrosting of low temperature coils which is ofsimple and inexpensive construction and is entirely reliable inoperation.

A feature of the invention is the provision of a defrosting systemincluding defroster tubes in conducting engage- .ment with therefrigerating coil structure, with the de- :froster tubes and a heatingchamber for heating fluid being provided as a sealed unit which isevacuated to the extent that a negative pressure exists therein so thatthe 'heated defrosting fluid initially moves rapidly through the coil.The fluid may be heated to a high temperature so that a large amount ofheat is provided for rapid defrosting, and the pressure within thesealed defrosting system may rise to positive pressures.

Another feature of the invention is the provision of a sealed defrostingsystem for a refrigeration coil structure including a heating chamberfor a vaporizable heating fluid, with a header extending vertically fromthe heating chamber and defroster tubes extending from the header, inwhich the header is of suflicient cross section to form a vapor dome forseparating vapor from liquid, and includes a chamber extending above theuppermost defroster tube for receiving foreign non-condensable gases.

A further feature of the invention is the provision of a defrostingsystem wherein a vaporizable fluid is heated in a heating chamber, withheating tubes sealed in the chamber and heating units provided in thetubes to be completely sealed from the circulating system for the iceheating fluid so that the pressure in this system may be controlled asdesired.

A still further feature of the invention is the provision of adefrosting system for a refrigerating coil wherein a vaporizable fluidis heated by electrical heating units and the entire structure is simpleand compact with the electrical connections being sealed from moistureresulting from the defrosting action.

Further objects, features and the attending advantages of the inventionwill be apparent from a consideration of the following description whentaken in connection with the accompanying drawings wherein:

Fig. 1 is an elevational view of a refrigerating coil structureincluding a circulating fan;

Fig. 2 is a cross section of the structure of Fig. 1;

Figs. 3, 4 and 5 show in detail the construction of the refrigerationcoil structure andthe defrosting system in accordance with theinvention;

Fig. 6 is a schematic diagram illustrating the operation of thedefrosting system in accordance with the invention; and

Figs. 7, 8, 9 and 10 illustrate an embodiment of the defrosting systemsuch as may be used for extremely large refrigerating coils.

In practicing the invention, a defrosting system is provided for arefrigeration coil structure which includes coil portions and finssecured thereto. The defrosting system includes a heating chamber with aheader extending vertically therefrom and defroster tubes connected tothe header and in contact with the fins of the refrigeration coilstructure. The defrosting system is a sealed unit which has therein aheating fluid which may be a suitable tion coil and a heater such as anelectric heater may beupright typesuch as might be used in arefrigerated cabi;

vaporizable liquid. The header has a cross section of such size to forma vapor dome which separates vapor from liquid so that the vapor risesand passes through the defroster tubes. The header has a chamber at thetop for receiving foreign gases which may be present in the sealed unit.For heating the vaporizable liquid, heating tubes are sealed in theheating chamber in which heating units may be provided so that theheating units are completely isolated from the heating fluid in thechamber. Provision is also made for sealed electrical connections forthe heating coils so that they are not affected by moisture produced bythe refrigeration and defrosting action. The defrosting system isevacuated so that a negative pressure is provided therein when theheating fluid is not heated. Accordingly, as the fluid is heated, thevapor moves through the defrosting coils very rapidly to immediatelystart the defrosting action. Suflicient heat may be provided to raisethe pressure in the sealed defrosting system to a positive pressure sothat highly efficient heating of the refrigeration coil is provided andthe defrosting may be very quickly accomplished. A control system may beprovided which turns off the compressor for the refrigerating system andturns on the defrosting system. A drain pan may be required under therefrigeraprovided for the drain pan. In the event that a fan is providedfor circulating air through the refrigerating coil, this fan may also beturned off during the defrosting cycle. A pressure limit valve may beprovided in the defrosting system to prevent the refrigerant from beingadmitted into the refrigerating coil during the defrosting cycle tofurther expedite the defrosting action. The control of the variouselements of the system may be provided by a timer so that the defrostingcycles occur at predetermined set times, and last for durations whichare set to, take care of particular situations.

Referring now to the drawings, Figs. 1 and 2 illustrate a refrigerationunit which may be used in various types of refrigerated cabinets. Thisunit is of the mullion or net for bakery goods. A vertical housing 10 isprovided having an intake opening 11 at the top through which air isdrawn by a circulating fan 12. As shown in Fig. 2, the air drawn in bythe fan is directed downwardly through the housing 10 across therefrigeration coil gen erally indicated as 13. The air is thendischargedat the bottom of the housing,,being directed in either or bothdirections through the openings 15 and/or 16. The coil unit 13 includesan intake coupling 17 for connection to thevoutput of a condenser, andan output coupling 26 for connection to a compressor of therefrigerating system.

In Figs. ,2, 3, 4 and the detailed construction of the refrigerationcoil and of the defrosting system are shown. The refrigeration fluidcomes. from inlet 17 through expansiorrvalve 18 to distributor 19 andthen to the two separate lines 2% and 21 (Fig. 3). The refrigerant isapplied through the coil structure including the longitudinal extendingtubes 22 (Fig. 5), the front end bends 23 (Fig. 3)., and the rear endbends 24 (Fig. 2). As is apparent from Fig. 4, vertically positionedfins 25 are provided having therein openings for receiving the variouslongitudinal tube portions 22 of each vertical row. The refrigerationfluid, which has absorbed heat and is now a vapor, is discharged throughreturn line 26. A bulb 27 connected to the expansion valve 18 isprovided on the output coupling '26.

The defrosting system includes a heating chamber or boiler 28 to whichis connected a header 29. Connected tothe header 29 are defroster tubes30 which are terminated at the rear end of the unit by the ductstructure 31. The rear duct structure includes a return section 32 whichmay extend under or around the refrigeration unit and is connected tothe lower part of the heating chamber 28. The vapor is condensed in thedefroster tubes 30 and returns by gravity through the duct structure 31to the chamber '23. It is to be noted that one coil portion 30 extendsin each vertical row of the longitudinal tubes of the refrigeration coilstructure and is in heat conducting relation with the fins thereof. Itis to be pointed out that in some systems the return duct structure maynot be necessary as the condensed vapor may return through the headerwith the rising vapor.

The chamber 28 has a recessed front portion 33 from which extend tubes34. These tubes are sealed to the end plates of the heating chamber sothat the space in the heating chamber, the header, defroster tubes andrear duct structure may be hermetically sealed as a closed system.Heating units may be provided in the tubes 34 and a safety thermostatmay also be provided in one of these tubes. The heating units and/orthermostat are connected to conductors provided with insulation suitableto withstand the moisture conditions encountered. The front end of therecess of the heating chamber 28 includes threaded lug portions 36 forconnection of a cover plate 37 thereon so that a moisture proofenclosure is provided for the electrical connections. This is quiteimportant since condensation and moisture from the de frosting of therefrigeration coils adversely affects the insulation on the conductorsand causes deterioration thereof. By providing the conductors throughthe waterproof conduit 35 and sealing the recess portion 33 of theheating chamber the connections may all be protected from such moisture.

Considering now the construction and operation of the defrosting system,a vaporizable fluid is provided in the heating chamber 28. A fluid suchas trichlorethylene in the pure state or Freon No. 113 may be suitablefor such use. The amount of fluid used is not critical but is preferablysuch that it completely covers the tubes 34 in the heatingchamber 28when the system is operating and some of the fluid is in liquid form. inthe defroster tubes. The fluid mayxcompletely till. the heating chamberand extend. up into the header when the fluid is cold and there is noliquid fluid in the defroster tubes. The sealed dc expansion valve.

frosting system is then evacuated so that a negative pressure isprovided in the sealed system. This may be accomplished by boiling thedefrosting liquid and then evacuating air from the filler tube 40 at thetop of the header 2.9. By way of illustration, the filler tube is shownhaving a valve 41 therein to permit the system tobe opened to providethe fluid therein, and for evacuating air therefrom.

It has been found that the system operates satisfactorilywhen a negativepressure is provided so that the absolute pr ssure within the system isof the order of 2 lbs. per square inch when using heating fluids asmentioned above. When the defrosting system is operated the heatingunits within the tube 34 are energized so that the fluid in the chamber28 boils. Because of the negative pressure the vapor rises through theheader 29 and then rapidly flows through the coil portions 30 into thereturn duct struc ture 31. As the heating continues, the pressure maybuild up within the system so that the system changes from a' vaporsystem to a positive pressure system. Pressures up to 15 lbs. per squareinch, or an absolute pressure of 30 lbs. per square inch may be used.The use of such a wide pressure change results in correspondingly hightemperature changes to thereby provide eflicient heating of the coil andrapid defrosting action. As an example, the coil may be operated at atemperature of minus 30 Fahrenheit and in such case the heater may heatthe fluid to a temperature of around 200 Fahrenheit. This results invery fast defrosting of the refrigeration coils.

It will be apparent from Fig. 5 that the header 29 is of relativelylarge diameter. This is necessary so that the vapor from the boilingfluid rises and passes to the defroster tubes and the liquid fluid doesnot rise but remains in the heating chamber. tends substantially abovethe uppermost defroster tube 30. This provides a chamber into whichnon-condensible foreign gases and air which may accumulate in the systemmay rise during the defrosting cycle. Accordingly, any foreign gas inthe system will have a minimum effect since it will not be in thecirculating path during the defrosting operation. Although it isdesirable that all air be removed from the system, this may not beentirely practical and the chamber at the top of the header renders thisless critical.

It will be noted that the unit as shown in Figs. 3 and 5 includes afront top bracket 42 for mounting the coil structure in the housing 10.This bracket together with the housing forms a substantially enclosedchamber at the front end of the structure so that heat from the header29 is effective to heat the return bends 23 of the refrigeration coilstructure to defrost the same. This increased defrosting action at thereturn bends heats the refrigerant therein to speed up defrostingaction. Similarly, at the rear of the unit the bracket 41 provided forsupporting the unit from the housing provides a substantially closedchamber at the back so that the heated defrosting fluid in the returnduct structure 31 heats the return bends 24 at the rear of the coil.

As shown in Fig. 2 a drip pan is provided at the bottom of the unit forreceiving moisture resulting from the defrosting action. The drip panhas an outlet for discharging such moisture into a sewer or the like.Heating units may be provided in the drip pan so that the very coldwater and ice resulting from the defrosting action is discharged throughthe outlet. Electrical heating units 45 may be provided for heating thedrip pan 44.

Considering now the operation of the complete refrig-- eration anddefrosting system, reference is made to Fig. 6. In this figure anelectric motor 56 drives compressor 51 which provides compressed fluidto the condenser 52. The condensed refrigeration fluid is applied to areceiver 54 from which it is coupled through the inlet line-l7 to the Adistributor may be provided for dividing the fluid into a. plurality ofchannels if this is desired. The refrigerating fluid flows through therefrig- The header 29 also ex-' pressed for repeated use. A fan 60 maybeprovided for circulating air through the refrigeration coil 22so thatthe cooled air may be used as desired. The refrigerating coil structureand the defrosting system are the same as in Figs. 1 to 5 and the samereference characters are used.

In Fig. 6, the compressor is shown energized from a 230 volt line, withthe connection being established through switch 61. The fan 60 isenergized from a 115 volt line, with the connection being establishedthrough a switch 62. Switches 61 and 62 may be controlled by a timer 63so that at a predetermined time or timm the refrigeration system isdeactivated and the defrosting system is rendered active. The timer mayfirst cause switch 61 to move to the upper position so that compressormotor 50 is stopped and the heaters in the heating chamber 28 areenergized. The vaporized defrosting fluid will then rise in the header29 and pass through the coils 30 in heat conducting relation with finsconnected to the refrigeration coils. At the same time that switch 61 isoperated, or at a somewhat later time, switch 62 will be operated tomove to the upper position so that the fan 60 is stopped and the heaters45 in the drip pan 44 are energized. As previously stated, this allowsthe defrosted moisture from the refrigerating coils to be held in aliquid stage so that it can be discharged from the drip pan. After apredetermined time, which has been determined to be adequate forremoving the ice from the refrigeration coils, the switch 61 again isoperated to the position as shown for energizing the compressor motor 50and for deenergizing the heaters in the heating chamber 28. At asomewhat later time the switch 62 may be released so that the fan 60 isagain operated and the drain pan heaters are de-energized.

The expansion valve 56 may be of a type which provides a shut-off whenthe pressure reaches a predetermined value. Accordingly, as the coil isheated and defrosted, the valve closes to prevent additionalrefrigeration fluid from flowing into the refrigeration coil. Thisspeeds up the defrosting action since the additional fluid does not needto be warmed up. This valve may operate automatically so that after thedefrosting action has started the valve will close. However, when thedefrosting cycle is over and the heater for the defrosting fluid isturned off, the expansion valve will again open to allow therefrigeration fluid to pass therethrough and to expand and cool therefrigeration coil in the normal way.

InFigs. 7 to l0 there is illustrated a defrosting system which issuitable for use in larger refrigeration units. In such units theheating chamber must be quite large and cannot conveniently be providedat one end of the coil. Accordingly, a relatively long inclined heatingchamber 65 is provided. A front header 66 is connected to the heatingchamber 65, being generally similar to the header 29 of thefirstembodiment. This header has a cross section sufliciently large thatliquid will not rise therein so that the vapor is effectively separatedfrom the liquid. Coil portions 67 extend from the header to carry vaporfrom the heated fluid in chamber 65 to a rear header 68. Both the frontand rear headers extend above the uppermost coil portion 67 to form achamber in which foreign non-condensible gasses collect as described inthe former embodiment. A return duct 69 connects the rear header 68 tothe heating chamber 65.

The heating chamber 65 may have a recess 70 at the lower end thereof,with heating tubes 71 extending from the recess as in the priorconstruction. Electrical connections are made within the recess, with aconduit 72 providing a waterproof connection thereto. In the cmw re 6bodiment of Figs. 7 to 10, a drip pan 73 may be provided under the coilhaving an outlet duct 74. This pan will be heated to a certain extent bythe radiated heat from the heating chamber 65, and then further heatedby a heating coil 75 provided on the bottom surface thereof.

The operation of the system of Figs. 7 to 10 is gen erally similar tothat of the prior embodiment. Be cause of the size of the boiler, theboiler heaters may be turned off earlier and the defrosting willcontinue for a time from the heat built up therein. This will cause theheating to drop off gradually so that the load on the compressor will beless when the refrigeration unit is started up again. In small unitssuch as in the prior embodiment it is not necessary to take theseadditional precautions.

The defrosting systems in accordance with the inven tion have been foundto be highly satisfactory in applications in which they have been used.Since the systems start as vacuum systems, the heated vapor willcirculate very fast so that the defrosting action will startimmediately. By continuing the heating until a relatively hightemperature is reached, ice built up on the refrigeration coils isremoved very quickly. As stated above, since the unit is sealed, heatingmay be applied until a positive pressure is provided in the system. Thisprovides a wide range operation and makes for very eflicient defrosting.

In representative systems the defrosting cycle will take place two orthree times in each 24 hour day. These times can be set so that theyinterfere as little as possible with the operation of the unit. Forexample, when used in a store, the defrosting timer may be set sothatdefrosting occurs when the store is closed and the refrigerator will notbe opened. The duration of the defrosting cycle may generally be of theorder of ten to twenty minutes depending upon the particularapplication. The number of cycles per day and the duration of the cyclescan of course be adjusted by setting the timer so that best operationwill be provided.

As a safety measure a thermostatic switch may be provided in the heatingchamber to out off the system in the event that the heating chamberreaches a dangerously high temperature. This thermostat may be put inone of the sealed tubes in the heating chamber in the same manner as aheating unit. However, during normal operation the thermostatic switchwould not operate since it is used only as a safety measure rather thanto stop the defrosting cycle. Although it is believed desirable to startand stop the cycle by a. timer providing predetermined time intervals,it is also possible to use other controls which respond to the conditionof the refrigeration coil structure.

Two embodiments of the invention have been illustrated which have beenfound desirable for units of two different sizes. Other configurationsmay be suitable for other applications incorporating the novel featuresset forth. The use of a sealed system operating at a negative pressurewith a header providing a vapor dome effect produces rapid circulationof the heating vapor through the system, and by heating the heatingfluid to a high tempera ture the defrosting action is quicklyaccomplished.

We claim:

1. A defrosting system for a refrigeration coil structure including incombination, a heating chamber for containing a heating fluid, a headerextending upwardly from said chamber, elongated defroster tubesextending substantially horizontally from said header and extendingthrough said refrigeration coil structure, and return duct structureconnecting said defroster tubes at the ends. thereof remote from saidheader to said heating chamber; said heating chamber, said header, saiddefroster tubes and said return duct structure being interconnected as ahermetically sealed unit; vaporizable heating fluid in said sealed unit;heating means in said heating chamber for heating the fluid therein;said hermetically sealed unit.

7 being evacuated so that substantially only the vapor pres.- sure ofthe fluid remains therein when said heating units are de-energized,whereby the vapor resulting from the heated fluid produced uponenergizing of said heating means flows rapidly through said defrostingcoil; said header extending above the uppermost defroster tube to form achamber for arresting any entrained foreign gas, entrapped in saidsealed unit.

2. A defrosting system for a refrigeration coil structure including incombination, a heating chamber for containing a heating fluid, a headerextending upwardly from said chamber, elongated defroster tubesextending substantially horizontally from said header and extendingthrough said refrigeration coil structure, and return duct structureconnecting said defroster tubes at the ends thereof remote from saidheader to said heating chamber; said heating chamber, said header, saiddefroster tubes and said return duct structure being interconnected as ahermetically sealed unit; vaporizable heating fluid in said sealed unit;heating means in said heating chamber for heating the fluid therein;said hermetically sealed unit being evacuated so that a negativepressure is provided therein when said heating means is de-energized,whereby the vapor resulting from heating of the vaporizable fluid risesin said header and flows rapidly through said defrosting coil; saidheader having a cross section of such size that a vapor dome is formedtherein which separates the heating fluid in vapor form from the heatingfluid in liquid form and the fluid in liquid form remains in saidheating chamber.

3. A defrosting system for a refrigeration coil structure including incombination, a heating chamber for containing a heating fluid, a headerextending upwardly from said chamber, elongated defroster tu-besextending substantially horizontally from said header and in heatconducting relation with said refrigeration coil structure, and returnduct structure connectingsaid defroster tubes at the ends thereof remotefrom said header to said heating chamber; said heating chamber, saidheader, said defroster tubes and said return duct structure beinginterconnected as a hermetically sealed unit; vaporizable heating fluidin said sealed unit; heating means in said heating chamber for heatingthe fluid therein; said hermetically seated unit being evacuated so thata negative pressure is provided therein when said heating means isde-energized, with operation of said heating units vapor-' izing saidheating fluid and providing increased pressure in said sealed unit; saidheader having a cross section of such size that a vapor dome is formedand vapor resulting from heating of said vaporizable fluid rises in saidheader and flows through said defroster tubes with the fluid in liquidstate remaining in said heating chamber.

4. A defrosting system for a refrigeration coil structure including incombination, a heating chamber for containing a vaporizable defrostingfluid, a header extending upwardly from said chamber, elongateddefroster tube extending substantially horizontally from said header andextending through said refrigeration coil structure, and return ductstructure connecting said defroster tubes at the ends thereof remotefrom said header to said heating chamber; said heating chamber, saidheader, said defroster tube and said return duct structure beinginterconnected as a hermetically sealed unit; vaporizable heating fluidin said sealed unit; a moisture-proof enclosure at one end of saidchamber, heating tubes extending from said enclosure into said heatingchamber, electrical heating units in said tubes for heating said fluidin said chamber, electrical connecting means in said enclosure forenergizing said heating units to heat said chamber and said heatingfluid; said hermetically sealed unit being evacuated so that a negativepressure is provided therein when said heating units are de-energized,whereby the vapor resulting from the heated fluid produced uponenergizing of said heating means flows rapidly 8 through said defrostertubes; said header extending above the uppermost defroster tube to forma chamber for con taining foreign gases entrapped in said sealed unit.

5. A defrosting system for a refrigeration coil structure which includesa plurality of coil portions through which a refrigerant is circulated,said defrosting system including in combination, defroster tubes havingportions in heating conducting relation with refrigeration coilstructure, a heating chamber for containing a vaporizable heating fluid,said defroster tubes and said heating chamber being interconnected as ahermetically sealed unit, heating fluid in the sealed unit, sealedheating units in said heating chamber for heating said fluid in saidchamber, control means including timing means for energizing saidheating units to heat said chamber and said defrosting fluid therein,said hermetically sealed unit being evacuated so that a negativepressure is provided therein when said heating units are de-energized,whereby the vapor resulting from heated fluid produced upon energizingof said heating means flows rapidly through said defroster tubes, saidcontrol means causing said heating means to be energized for asufficient length of time that the heating fluid expands and produces apositive pressure in said sealed unit to provide rapid heating of therefrigerating coil structure to defrost the same, and valve means forcontrollingflow of refrigerant through the refrigeration coil portions,said valve means including means responsive to the pressure of therefrigerant for stopping the flow of refrigerant when the pressurethereof reaches a predetermined value resulting from heating of therefrigerating coil structure.

67 A defrosting system for a refrigeration coil structure including incombination, a valve for controlling 'the flow of refrigerant in thecoil structure, defroster tubes having portions in heat conductingrelation with the refrigeration coil sttructure, a heating chamber forcontaining a heating fluid, said defroster tubes and said heatingchamber being interconnected as a hermetically sealed unit, vaporizableheating fluid in the sealed unit, heating means extending into saidheating chamber for heating said fluid therein, said hermetically sealedunit being evacuated so that a negative pressure is provided thereinwhen said heating units are de-energized, whereby the vapor resultingfrom the heating fluid produced upon energizing of said heating meansflows rapidly through said defrosting coil, said heating means whenenergized causing the heating fluid to be heated to an extent to providerapid heating of the refrigeratingcoil structure to defrost the same,said valve including means responsive to the pressure of the refrigerantfor stopping the flow of refrigerant through the refrigeration coilstructure when the pressure of the refrigerant reaches a predeterminedvalue resulting from the heating of the refrigerating coil structure.

7. A defrosting system for a refrigeration coil structure including incombination, a heating chamber for containing a heating fluid, a headerextending upwardly from said chamber, elongated defroster tubesextending substantially horizontally from said header and extendingthrough said refrigeration coil structure, said heating chamber, saidheader, and said defroster tubes being interconnected as a hermeticallysealed unit; vaporizable heating fluid in said sealed unit, heatingmeans in said heating chamber for heating the fluid therein, saidhermetically sealed unit being evacuated and being substantially free ofair and foreign gases and having a negative pressure therein when saidheating means is deenergized, whereby the vapor resulting from heatingof the vaporizable fluid rises in said header and flows rapidly throughsaid defrosting coil; said header having a cross section of such sizethat a vapor dome is formed therein which separates the heating fluid invapor form from the heating fluid in liquid form and the fluid in liquidform remains in said heating chamber, said header extending above theuppermost defroster tube to form a chamber for holding any entrained airor foreign gases out of the 2,181,276 Kogel et a1 Nov. 28, 1939 path ofcirculation of the vaporized heating fluid. 2,526,032 La Porte Oct. 17,1950 2,551,163 Riekert et a1 May 1, 1951 References Cited in the file ofthis patent 2,553,657 La Porte May 22, 1951 5 2,652,697 Pellegrini Sept.22, 1953 UNITED STATES PATENTS 2,081,479 Fink May 25, 1937

